Process for passivating sulfidic iron-containing rock

ABSTRACT

A method is provided for passivating sulfidic iron-containing rock comprising contacting sulfidic iron-containing rock with one or more members of the group consisting of magnesium oxide, magnesium hydroxide, magnesium chloride, magnesium nitrate and magnesium carbonate, thereby reducing the acid generation potential of rock.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. provisional applicationSerial No. 60/304,599, filed Jul. 10, 2001, which is hereby incorporatedby reference in its entirety to the extent not inconsistent with thedisclosure herewith.

BACKGROUND OF THE INVENTION

[0002] This invention is in the field of reducing or eliminating acidrock drainage from sulfidic iron containing rocks and acidic mine wastetailings. Acid rock drainage (formation of sulfuric acid and relatedacids from natural air/water oxidation processes on various materials)is a common phenomenon from mining and leaching of various metallic andnon-metallic minerals such as iron-containing sulfidic materials. Thesesulfidic materials include tailings, overburden, discarded waste rockand unmined exposed rock. Acid rock drainage causes severe pollutionproblems throughout the world.

[0003] There have been various attempts to render these sulfidicmaterials non-reactive which include partially converting pyrite andpyrrhotite into an oxide structure so that each iron sulfide particle iscoated with an iron oxide film, microencapsulation of pyrite byartificial inducement of iron phosphate coatings, the coating of exposedsurfaces with various polymeric materials, and the formation ofmanganese dioxide coatings on pyrite surfaces. These methods of treatingmaterials have been at best partially effective and economicallyunattractive.

[0004] U.S. Pat. No. 5,587,001 (DeVries, Dec. 24, 1996) describes amethod for reducing acid rock drainage from sulfidic iron-containingrock by contacting the rock with an aqueous solution of manganate ion ata pH between 6-13. This treatment reportedly creates a manganese oxidelayer on the iron-containing sulfidic rock. The process in the U.S. Pat.No. 5,587,001 requires pH 6-13 at all times during the treatment,preferably a pH greater than 10. U.S. Pat. No. 5,587,001 also requiresthat permanganata color be maintained during the treatment. Thiscondition often requires high dosage of manganate ions for treatingreactive tailings because a considerable amount of manganate ions aredissolved in solution and react with other ions before reaching thesulfide surface. U.S. Pat. No. 5,587,001 also requires that the sulfidescontain a significant concentration of iron bearing minerals so that thereaction between iron bearing sulfides and permanganate ions can besustained. Several dissolved metals undergo precipitation reactions atpH>12. Precipitated metal hydoxycomplexes coat the sulfides, thuspreventing the desired electrochemical reaction.

[0005] U.S. Pat. No. 6,086,847 (Thompson, Jul. 11, 2000) discloses aprocess for reportedly preventing acid rock drainage of metal-bearingrocks comprising contacting a sulfidic iron-containing rock with an acidpassivating agent which comprises at least one alkaline earth metal toproduce a combination; contacting the combination with manganate ionsand a base and maintaining the pH of the system between 11 and 13.5.

[0006] The waste rock naturally yields very low acidic pH in the rangeof 1-4. To raise the pH and maintain it at a higher level than isnaturally found (such as the pH required by the process disclosed inU.S. Pat. Nos. 5,587,001 and 6,086,847) requires high dosage ofneutralization agents (for example, lime/caustic soda). This is noteconomically and technically viable. Also, at high pH (above about11.0), gypsum (CaSO₄) and MgSO₄ precipitate on the sulfide and completecoating of desired materials cannot be achieved. Improved and costeffective treatments are necessary to treat iron containing sulfidicminerals to prevent or minimize the natural oxidation of these materialsto form acids.

SUMMARY OF THE INVENTION

[0007] A method for passivating sulfidic iron-containing rock and minewastes is provided. This method is useful to reduce the amount of acidrock drainage from mine waste tailings and other areas where formationof acid products is a problem, among other uses. The process disclosedherein is independent of the concentration of iron sulfide in thematerials to be treated and the physical state of materials. Also, thelow pH treatment ensures that the sulfide surfaces are exposed and arein direct contact with the layers of coating agents.

[0008] The tailings, waste rock and other exposed surfaces at miningoperations can react with atmospheric air and surface water over aperiod of time forming polluting acid drainage. Formation of magnesiumoxysulfate coatings on iron-containing sulfides shield them fromatmospheric air or surface water containing oxygen to prevent orminimize acid drainage problems.

[0009] As used herein, “passivating” means rendering the substancepassivated less reactive than it was before passivation. For example, apassivated sulfidic iron-containing ore is an ore that generates no acidor less acid than a non-passivated ore upon being exposed toacid-generating and/or weathering processes. An “effective amount” is anamount that given the desired effect, as taught herein.

[0010] In one embodiment, the process of this invention for passivatingsulfidic iron-containing rock comprises the steps of:

[0011] contacting said rock with a magnesium-containing substance; ifnecessary, adjusting the pH of the slurry so that magnesium oxysulfateis formed; optionally adding silicates, for example, sodium or calciumsilicates; optionally allowing oxidation of the rock to form oxysulfateson a surface of the rock; and optionally adding an iron-containingsubstance, for example FeCl₃ or Fe₂(SO₄)₃ to form ferrous iron-magnesiumsulfates.

[0012] The magnesium-containing substance used can be any suitablecomposition such as one or more members of the group consisting ofmagnesium oxide, magnesium hydroxide, magnesium chloride, magnesiumnitrate and magnesium carbonate. In addition, any suitable form can beused. For example, an aqueous saturated solution may be used, or drysolid may be used. The use of magnesium hydroxide prevents reaching ofoverdose level of alkali. An overdose level of alkali is theconcentration that blocks solution passage and permeability. The use ofMgO maintains the pH at near 9 and below. Preferably, themagnesium-containing substance is in the form of an aqueous saturatedsolution of magnesium oxide or dry magnesium oxide (about 2.2-22.0 lbsMgO/ton of rock which is about 0.1-1% magnesium oxide by weight in thesolution) or magnesium hydroxide (preferably 2.5% by weight of solutionmagnesium hydroxide).

[0013] Preferably, the rock and magnesium are reacted in the form of aslurry. The rock can be directly treated in the natural environment oras crushed rock preferably containing about 20%-50% by weight of solids,but any concentration or range of concentrations which allows thedesired reaction to occur at a desired rate is included in thisdescription. When magnesium oxide is used, the weight ratio of magnesiumoxide: rock: water is preferably maintained at up to1:100:400-10:100:400.

[0014] The pH of the rock slurry is usually between about 1 and 5 as itnaturally occurs. It is generally not necessary to adjust the pH of theslurry before treatment. If the pH of the starting system is greaterthan about 4-5, pH adjustment is needed using any suitable pH adjustmenttreatment, as described further herein and as is known in the artwithout undue experimentation.

[0015] After the magnesium-containing substance is contacted with therock for a time sufficient to form magnesium sulfate as determined bymeans known in the art, the pH is raised by any means known in the art(preferably calcium oxide or sodium hydroxide are added) to causes theformation of magnesium oxysulfate (preferably the pH is raised to 9-11for the formation of magnesium oxysulfate). At this point, the reactivesulfide in the rock is stabilized. Optional oxidation of the slurry,preferably with air, but any oxidizing agent may be used, results in theformation of different phases of oxysulfates on the surfaces of thesulfides. If desired, an effective amount (for example, 1-5 lb/ton rock)of silicate (for example, sodium silicate or calcium silicate) added atany stage of the process increases the strength of the coating due toformation of magnesium sulfate and magnesium silicate compounds. Anyamount of silicate that causes formation of magnesium sulfate ormagnesium silicate can be used. If desired, an iron-containing substancesuch as an iron salt may be added in a suitable concentration to formferrous iron-magnesium sulfates. The iron salt may be any suitable saltknown to one of ordinary skill in the art, including FeCl₃. Theconcentration of iron-containing substance added is any concentrationsufficient to form the desired amount of ferrous iron-magnesium sulfate.If there is a low concentration of dissolved iron, enough iron must beadded to form the complex. This is typically 1-2 lb/ton of ore.

[0016] A presently-preferred embodiment of the process is the method ofreducing acid rock drainage from sulfidic iron-containing rockcomprising the steps of contacting said rock with dry/hydrated magnesiumoxide wherein the concentration of magnesium oxide in the mixture is0.1-1% by weight and the slurry density is about 20% by weight of solidsin the mixture, and the pH of the resultant slurry is between 1-5;allowing a reaction between magnesium oxide and the sulfides in saidrock to proceed so as to form in slurry dissolved magnesium sulfate;raising the pH of the slurry to form magnesium oxysulfate (preferably bythe addition of CaO or sodium hydroxide, and preferably to about10-10.5); optionally adding silicates of sodium or calcium; optionallyperforming air oxidation of slurry so as to cause the formation ofmagnesium oxysulfates coating on the surface of said sulfides.

[0017] Another preferred embodiment of the process is a process forreducing acid rock drainage from sulfidic iron-containing rockcomprising the steps of:

[0018] contacting said rock with an aqueous colloidal suspension of 2.5%magnesium hydroxide; allowing a reaction between magnesium hydroxide andthe sulfides in said rock to proceed; raising the pH of the slurry toform magnesium oxysulfate (preferably by the addition of CaO or sodiumhydroxide, preferably to about 10-10.5); optionally adding silicates ofsodium or calcium; optionally performing air oxidation of slurry;optionally adding FeCl₃ or other iron salts.

BRIEF DESCRIPTION OF THE FIGURE

[0019]FIG. 1 shows solution pH of pyrite sample in the hydrogen peroxidetest as a function of time after passivation using MgO and silicate.

DETAILED DESCRIPTION OF THE INVENTION

[0020] The ores that may be treated using the method of the inventioninclude pyrrhotite, bomite, chalcopyrite, arsenopyrite and pyrite. Anyore that contains iron and sulfur in its reduced form (sulfide) may betreated to passivate the sulfur using the disclosed process. The ore maybe in any form, for example, slurry, rock pile or exposed rock.

[0021] The reaction proceeds for a suitable time required to achieve thedesired amount of passivation of the sulfur in the ore. This timenaturally depends on the nature of the ore treated, the desired amountof passivation of the sulfur in the rock and other parameters, such asconcentration of reactants used. This time is readily determined byroutine experimentation well within the skill of one of ordinary skillin the art without undue experimentation, using the teachings herein.

[0022] The processes of this invention can be carried out attemperatures above the freezing point of the solutions up to about 60°C.

[0023] Applicant does not wish to be bound by any theory presentedherein. The theory and examples below are presented to aid in theunderstanding of the invention and illustrating some of thepresently-preferred embodiments of the invention.

EXAMPLE 1

[0024] Effect of Magnesium Oxide Dosage on Passivation in the Presenceof Permanganate

[0025] 5 gms −325 mesh pure pyrite sample was mixed with 20 mg of limeto increase the pH to basic pH (about 10.5). In place of lime, causticsoda or sodium carbonate or other suitable materials that increase thepH to the desired range may be added. Different dosage levels ofmagnesium oxide were used (0, 2.2 lbs magnesium oxide/ton rock, 4.4lbs./t, 8.8 lbs./t, 13.2 lbs./t and 22.0 lbs./t). 20 ml. of tap waterwas added to the mixture of pyrite, lime and magnesium oxide and theslurry pH were measured to be about 1.5. The slurry pH was then raisedto 10-10.3 by the addition of 1 N NaOH. At this point, 1.32 lbs./t ofpermanganate was added. The slurry was left undisturbed for 2 hours. Theslurry was filtered and the solids were washed. The washed solids weresuspended in 91 ml. of water and to this 9 ml. of 50% hydrogen peroxidewas added. The pH of the solution was monitored for 1 day. At the end of1-day duration, the tests which showed pH of above 7, were considered tobe successful tests in-terms of passivation. If the pH drops below 7much before 24 hours, the test is also considered a fail. The resultsare presented in Table 1 below. TABLE 1 Peroxide Tests Results withdifferent dosage of MgO. The KMnO₄ dosage was maintained constant ineach test (1.32 lbs./T) MgO Dosage Peroxide (lbs./t) Test Result Remarks0 Failed Vigorous reaction, fails at 60 minutes 2.2 Failed Vigorousreaction, fails at 60 minutes 4.4 Failed Vigorous reaction, fails at 60minutes 8.8 Failed Slow reaction, fails after 1 day 13.2 Failed Slowreaction, fails after 1 day 22.0 Passed Slow reaction, pH above 7.8

[0026] These results show that at lower dosages of MgO, passivation wasnot effective due to enormous surface area of pyrite involved. However,when the dosage was increased to 22 lbs./t level, the pyrite wassuccessfully passivated. Considering the fact that in mine tailingssample, the pyrite present is fraction of the total sample, the dosagelevel of MgO required to passivate an actual sample will be atconsiderably lesser dosage level than 22 lbs./t.

[0027] These results show that in order to passivate the same pyritesample, MgO dosage level of 22 lbs./t was required. Note that in theseexperiments, permanganate dosage level of 1.37 lbs./t was present. Sincepermanganate is beneficial in passivating the pyrite sample (asindicated in U.S. Pat. No. 5,587,001), it was not clear as to whatextent MgO was responsible for the passivation.

EXAMPLE 2

[0028] Effect of Potassium Permanganate Dosage on Passivation in thePresence of MgO

[0029] 5 gms −325 mesh pure pyrite sample was mixed with 10 mg of MgOand 20 mg of lime. This amounts to 4.4 lbs./t of MgO and 8.8 lbs./t ofCaO. 20 ml. of tap water was added to the mixture of pyrite, lime andmagnesium oxide and the slurry pH were measured to be about 1.5. Theslurry pH was then raised to 10-10.3 by the addition of 1 N NaOH. Atthis point, different dosage of permanganate (0, 1.32 lbs./t, 2.64lbs./t, 5.28 lbs./t, 10.56 lbs./t, 21.12 lbs./t) was added. The slurrywas left undisturbed for 2 hours. The slurry was filtered and the solidswere washed. The washed solids were suspended in 91 ml. of water and tothis 9 ml. of 50% hydrogen peroxide was added. The pH of the solutionwas monitored for 1 day. At the end of 1-day duration, the tests whichshowed pH of above 7, were considered to be successful tests in-terms ofpassivation. The results are presented below in Table 2. TABLE 2Peroxide Tests Results with different dosage of KMnO₄. The MgO dosagewas maintained constant in each test (4.4 lbs./T) KMnO₄ Peroxide Dosage(lbs./t) Test Result Remarks 0 Failed Vigorous reaction, fails at 60minutes 1.32 Failed Vigorous reaction, fails at 60 minutes 2.64 FailedVigorous reaction, fails at 60 minutes 5.28 Failed Vigorous reaction,fails at 60 minutes 10.56 Failed Vigorous reaction, fails after 1 day21.12 Passed Slow reaction, pH above 9

[0030] These results show that in order to passivate the same pyritesample, permanganate dosage level of 21.12 was required. Please notethat in these experiments, MgO dosage level of 4.4 lbs./t was present.Since MgO is beneficial in passivating the pyrite sample, it was notclear as to what extent permanganate was responsible for thepassivation.

EXAMPLE 3

[0031] Effect of Magnesium Oxide Dosage on Passivation in the Absence ofPermanganate

[0032] 5 gms −325 mesh pure pyrite sample was used. Different dosagelevels of magnesium oxide were used (11.0 lbs./t, 15.4 lbs./t, 19.8lbs./t, and 22.0 lbs./t). 20 ml. of tap water was added to the mixtureof pyrite and magnesium oxide. The slurry pH was measured to be about1.3. The slurry pH was then raised to 10-10.3 by the addition of 1 NNaOH. The slurry was left undisturbed for 2 hours. The slurry wasfiltered and the solids were washed. The washed solids were suspended in91 ml. of water and to this 9 ml. of 50% hydrogen peroxide was added.The pH of the solution was monitored for 1 day. At the end of 1-dayduration, the tests which showed pH of above 7, were considered to besuccessful tests in-terms of passivation. The results are presented inTable 3 below. TABLE 3 Peroxide Tests Results with different dosage ofMgO in the absence of KMnO₄. MgO Dosage Peroxide (lbs./t) Test ResultRemarks 11.0 Failed Vigorous reaction, fails at 60 minutes 15.4 FailedVigorous reaction, fails at 60 minutes 19.8 Failed Vigorous reaction,fails at 60 minutes 22.0 Passed Slow reaction, pH above 7.5

[0033] Comparing the results of Table 1 and Table 3, it is clear thatthe presence of permanganate does not favorably affect the passivationprocess. To passivate the pyrite sample, 22.0 lbs./t of MgO was neededregardless of the presence of permanganate in the solution.

EXAMPLE 4

[0034] Effect of Potassium Permanganate Dosage on Passivation in theAbsence of MgO

[0035] 5 gms −325 mesh pure pyrite sample was used. 20 ml. of tap waterwas added to the pyrite and the slurry pH was measured to be about 1.3.The slurry pH was then raised to about 8 by the addition of 1 N NaOH. Atthis point, different dosage of permanganate (6.6 lbs./t, 11.0 lbs./t,13.2lbs/t, 15.4lbs./t and 22.0 lbs./t) was added. The final pH wasadjusted to be 10-10.3. The slurry was left undisturbed for 2 hours. Theslurry was filtered and the solids were washed. The washed solids weresuspended in 91 ml. of water and to this 9 ml. of 50% hydrogen peroxidewas added. The pH of the solution was monitored for 1 day. At the end of1-day duration, the tests which showed pH of above 7, were considered tobe successful tests in-terms of passivation. The results are presentedbelow in Table 4.

[0036] Comparing the results of Table 2 and Table 4, it is clear thatpermanganate dosage level about 15.4 lbs./t is needed in the absence ofMgO to passivate the pyrite. TABLE 4 Peroxide Tests Results withdifferent dosage of KMnO₄ in the absence of MgO KMnO₄ Peroxide Dosage(lbs./t) Test Result Remarks 6.6 Failed Vigorous reaction, fails at 60minutes 11.0 Failed Vigorous reaction, fails at 60 minutes 13.2 FailedVigorous reaction, fails at 60 minutes 15.4 Passed Slow reaction, pHabove 7 22.0 Passed Slow reaction, pH above 7

[0037] The results listed in Table 1-4 show that the passivation isfavorably affected by increasing the dosage level of permanganate andMgO. On a tonnage basis, even though little higher dosage of MgO isrequired than permanganate, however, considering the enormous pricedifference between permanganate and MgO (Permanganate $1.50/lb, MgO 50cents/lb), it is economical to use MgO in place of permanganate.

EXAMPLE 5

[0038] Effect of Lime Dosage on Passivation in the Absence of MgO

[0039] 5 gms −325 mesh pure pyrite sample was mixed with 100 mg of lime(44.0 lbs./t). 20 ml. of tap water was added to the mixture of pyriteand lime and the slurry pH were measured to be about 4.5. The slurry pHwas then raised to 10-10.3 by the addition of 1 N NaOH. At this point,1.32 lbs./t of permanganate was added. The slurry was left undisturbedfor 2 hours. The slurry was filtered and the solids were washed. Thewashed solids were suspended in 91 ml. of water and to this 9 ml. of 50%hydrogen peroxide was added. The pH of the solution was monitored for 1day. At the end of 1-day duration, the tests which showed pH of above 7,were considered to be successful tests in-terms of passivation. Theresults are presented in Table 5 below. TABLE 5 Peroxide Tests Resultswith High dosage of CaO in the absence of MgO The KMnO₄ dosage wasmaintained (1.32 lbs./T). CaO Dosage Peroxide (lbs./t) Test ResultRemarks 44.0 Failed Vigorous reaction, fails at 60 minutes

[0040] As expected even at very high dosage of CaO, the passivation didnot occur.

EXAMPLE 6

[0041] Effect of Addition of Magnesium Oxide at Higher pH (5.0) onPassivation in the Absence of Permanganate

[0042] 5 gms −325 mesh pure pyrite sample was mixed with 20 mg of CaO.20 ml. of tap water was added to the mixture of pyrite and lime. Theslurry pH was then raised to 5 by the addition of 1 N NaOH. At pH 5.0,22 lbs./t of MgO was added. The pH was then raised to 10-10.3. Theslurry was left undisturbed for 2 hours. The slurry was filtered and thesolids were washed. The washed solids were suspended in 91 ml. of waterand to this 9 ml. of 50% hydrogen peroxide was added. The pH of thesolution was monitored for 1 day. At the end of 1 day duration, thetests which show pH greater than 7 were considered successful in termsof passivation. The results are presented in Table 6 below. TABLE 6Peroxide Tests Results with MgO added at pH 5.0 in the absence of KMnO₄.MgO Dosage Peroxide (lbs./t) Test Result Remarks 22.0 Passed Slowreaction, pH above 7.5

[0043] Comparing the results of Table 3 and Table 6 it is clear that theaddition of MgO whether added at pH 1.7 or at pH 5.7 does not make anydifference.

EXAMPLE 7

[0044] Effect of the Addition of Hydrated Magnesium Oxide on Passivation

[0045] 5 gms −325 mesh pure pyrite sample was mixed with 20 mg of CaO.20 ml. of tap water was added to the 50 mg of MgO, which resulted in thepH of 10.3. This hydrated MgO slurry was added to the mixture of pyriteand lime. The slurry pH was then raised to 10-10.3 by the addition of 1N NaOH and 1.32 lbs./t KMnO₄ was added. The slurry was left undisturbedfor 2 hours. The slurry was filtered and the solids were washed. Thewashed solids were suspended in 91 ml. of water and to this 9 ml. of 50%hydrogen peroxide was added. The pH of the solution was monitored for 1day. At the end of 1 day duration, the tests which showed pH of above 7were considered to be successful tests in-terms of passivation. Theresults are presented in Table 7 below. TABLE 7 Peroxide Tests Resultswith the MgO addition in Hydrated Form MgO Dosage Peroxide (lbs./t) TestResult Remarks 22.0 Passed Slow reaction, pH above 7.5

[0046] Comparing the results of Table 3, Table 6 and Table 7 it is clearthat the pH and the form of MgO does not affect the passivation process.

EXAMPLE 8

[0047] MgO as Limiting Factor in the Passivation Process

[0048] 5 gms −325 mesh pure pyrite sample was mixed with 50 mg (22.0lbs./t) of MgO and 20 mg of CaO. 20 ml. of tap water was added to themixture of pyrite, lime and magnesium oxide and the slurry pH weremeasured to be about 1.65. The slurry was subjected to differenttreatments, such as pH adjustment to 10.0 and 12.0 followed by with andwithout aeration, KMnO₄ addition at pH 10.0 and 12.0 followed by withand without aeration, KMnO₄ addition at low pH followed by with andwithout aeration at pH 10.0. For the tests where there was no aeration,the slurry was left undisturbed for 2 hours. The slurry was thenfiltered and the solids were washed. The washed solids were suspended in91 ml. of water and to this 9 ml. of 50% hydrogen peroxide was added.The pH of the solution was monitored for 2 days. The tests which showedpH of above 7, were considered to be successful tests in-terms ofpassivation. The test conditions and results are presented in Table 8below. TABLE 8 Peroxide Tests Results with different conditions in thepresence of 22.0 lbs./t of MgO Peroxide Conditions Test Result pHmeasured after 2 days Adjusted to pH 10.0 Passed Final pH 7.62 Adjustedto pH 12.0 Passed Final pH 8.08 Adjusted to pH 10, Passed Final pH 7.652 hours of Aeration Adjusted to pH 10 + added Passed Final pH 7.82 1.37lbs./t of permanganate Adjusted to pH 10 + added Passed Final pH 7.651.37 lbs./t of permanganate, 2 hours of aeration Adjusted to pH 12 +1.37 Passed Final pH 8.05 lbs./t of permanganate Added 1.37 lbs./t ofPassed Final pH 7.59 permanganate at pH 1.7, Increase pH to 10 Added1.37 lbs./t of Passed Final pH 7.58 permanganate at pH 1.7 Increase pHto 10, 2 hours of aeration Added 1.37 lbs./t of Passed Final pH 7.85permanganate at pH 5.7, Increase pH to 10, 2 hours of aeration

[0049] The results listed in Table 8 clearly show that the addition ofMgO is a limiting factor in the passivation process. As long as the22-lbs./t-dosage level of MgO was met in the experiment, the passivationis successfully achieved in all the tests. However, the pH monitoringdata shows that the aeration is beneficial during the passivationtreatment and brings down the dosage level of MgO required to achievethe passivation.

EXAMPLE 9

[0050] Effect of Magnesium Oxide Dosage on Passivation for Hecla TailingSample

[0051] 5 gms of as-received dry Hecla tailings sample was mixed with 20mg of CaO and different dosage levels of magnesium oxide (0, 2.2 lbs./t,4.4 lbs./t, 8.8 lbs./t). 20 ml. of tap water was added to the mixture ofpyrite, lime and magnesium oxide. Hecla is a mine in Idaho. The slurrypH was measured to be about 12.02, 12.28, 12.3 and 12.4 respectively.The slurry was left undisturbed for 2 hours. The slurry was filtered andthe solids were washed. The washed solids were suspended in 91 ml. ofwater and to this 9 ml. of 50% hydrogen peroxide was added. The pH ofthe solution was monitored for 1 day. At the end of 1-day duration, thetests which showed pH of above 7, were considered to be successful testsin-terms of passivation. The results are presented in Table 9 below.TABLE 9 Peroxide Tests Results for the Hecla Tailings Sample withDifferent Dosage of MgO. MgO Dosage Peroxide (lbs./T) Test Result FinalpH after 1 day 0 Failed 4.3 2.2 Passed 7.28 4.4 Passed 8.03 8.8 Passed8.20

[0052] The data in Table 9 shows that much lower dosage of MgO (<2.2lbs./t) was required as opposed to 22 lbs./t in the case of pyrite.

EXAMPLE 10

[0053] Effect of Magnesium Oxide Dosage on Passivation for Nevada MineTailings Sample

[0054] 5 gms of as-received dry mine tailings sample from a mine inNevada was mixed with 20 mg of CaO and different dosage levels ofmagnesium oxide (0, 2.2 lbs./t, 4.4 lbs./t, 8.8 lbs./t, 13.20 lbs./t,17.60 lbs./t). 20 ml. of tap water was added to the mixture of pyrite,lime and magnesium oxide. The slurry pH was adjusted to 10.0 with 1 NNaOH. The slurry was left undisturbed for 2 hours. The slurry wasfiltered and the solids were washed. The washed solids were suspended in91 ml. of water and to this 9 ml. of 50% hydrogen peroxide was added.The pH of the solution was monitored for 1 day. At the end of 1-dayduration, the tests which showed pH of above 7, were considered to besuccessful tests in-terms of passivation. The results are presented inTable 10 below. TABLE 10 Peroxide Tests Results for Mine Tailings Samplewith Different Dosage of MgO. MgO Dosage Peroxide (lbs./T) Test ResultRemarks 0 Failed pH 2.54 after 3 hours 2.2 Failed pH 2.57 after 3 hours4.4 Failed pH 2.59 after 3 hours 8.8 Failed pH 3.58 after 3 hours 13.20Passed Final pH after 1 day 7.22 17.60 Passed Final pH after 1 day 7.42

[0055] The data in Table 10 shows that much lower dosage of MgO (<13.2lbs./t) was required as compared to 22 lbs./t in the case of pyrite.

EXAMPLE 11

[0056] Effect of Magnesium Oxide Dosage on Passivation for Ruby GulchTailings Sample

[0057] 5 gms of as-received dry Ruby Gulch tailings sample was mixedwith 20 mg of CaO and different dosage levels of magnesium oxide (0, 2.2lbs./t, 4.4 lbs./t, 8.8 lbs./t, 13.20 lbs./t). 20 ml. of tap water wasadded to the mixture of pyrite, lime and magnesium oxide. Ruby Gulch isa mining site in South Dakota. The slurry pH was adjusted to 10.0 with 1N NaOH. The slurry was left undisturbed for 2 hours. The slurry wasfiltered and the solids were washed. The washed solids were suspended in91 ml. of water and to this 9 ml. of 50% hydrogen peroxide was added.The pH of the solution was monitored for 1 day. At the end of 1-dayduration, the tests which showed pH of above 7, were considered to besuccessful tests in-terms of passivation. The results are presented inTable 11 below. TABLE 11 Peroxide Tests Results for the Ruby GulchTailings Sample with Different Dosage of MgO. MgO Dosage (lbs./T)Peroxide Test Result Remarks 0 Failed pH 3.16 after 3 hours 2.2 FailedpH 3.52 after 3 hours 4.4 Failed pH 6.34 after 1 day 8.8 Passed Final pHafter 1 day 7.17 13.20 Passed Final pH after 1 day 7.82

[0058] The data in Table 11 shows that much lower dosage of MgO (<8.8lbs./t) was required as opposed to 22 lbs./t in the case of pyrite.

[0059] A large column test was performed using magnesium oxide. The pHduring passivation was maintained at 10 using MgO only. MgO was added asa passivating agent. After passivation, a sample representing 150 gramsof solid was transferred to the humidity cell experiment. The humiditycell experiment was operated on seven-day cycles. In the first threedays dry air was passed into the sample, followed by three-daymoisturized air treatment. On the seventh day the sample was leached andthe leachate was analyzed for pH, alkalinity, acidity, sulfate and otherelements. Long-term testing with Ruby Gulch tailings affirmed theeffectiveness of the process, as shown in Table 12.

[0060] In the table below, each cycle is for the same sample and isreported as the function number of cycles. TABLE 12 Analysis ofleachates obtained from humidity cell experiments (Column test, RubyGulch - Waste Dump Sample, High Sulfide) Sample weight: 4000 g Dosage:7.7 lbs./t Magnesium Oxide Sample CYCLE-1 CYCLE-2 CYCLE-3 CYCLE-4CYCLE-5 CYCLE-6 Constituents (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) (mg/l)PH 7.63 8.05 7.61 8.31 8.32 8.09 Conductivity 920 890 460 370 225 220(μυ/cm) Acidity as 0 <15 <15 <15 <15 <15 CaCO₃ Alkalinity as 44 30 40 2545 30 CaCO₃ Calcium 19.8 18.9 16.9 13.8 11.1 11.4 Iron 0.028 0.051 0.030<0.020 <0.020 0.020 Magnesium 127 104 42.7 32.6 14.2 13.6 Manganese0.036 <0.010 <0.010 <0.010 <0.010 <0.010 Sulfate 492 139 95.6 28.4 26.327.8 TDS 700 560 235 238 175 <50 Antimony <0.003 <0.006 0.006 <0.006<0.006 <0.006 Barium <0.050 0.061 0.064 0.080 0.075 0.075 Beryllium<0.002 <0.002 <0.002 <0.002 <0.002 <0.002 Cadmium <0.002 <0.003 <0.003<0.003 <0.003 <0.003 Chromium <0.010 <0.010 <0.010 <0.010 <0.010 <0.010Cobalt <0.025 <0.025 <0.025 <0.025 <0.025 <0.025 Copper <0.010 <0.010<0.010 <0.010 <0.010 <0.010 Lead <0.007 <0.007 <0.007 <0.007 <0.007<0.007 Mercury <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 Molybdenum<0.050 <0.050 <0.050 <0.050 <0.050 <0.050 Nickel <0.025 <0.025 <0.025<0.025 <0.025 <0.025 Selenium <0.007 <0.007 <0.007 <0.007 <0.007 <0.007Silver <0.035 <0.035 <0.035 <0.035 <0.035 <0.035 Thallium <0.001 <0.001<0.001 <0.001 <0.001 <0.001 Vanadium <0.100 <0.100 <0.100 <0.100 <0.100<0.100 Zinc <0.050 <0.050 <0.050 <0.050 <0.050 <0.050

EXAMPLE 12

[0061] Combined Effect of Sodium Silicate with MgO

[0062] In another series of experiments the combined effect MgO withsilicate were tested. The pH of the pyrite sample was increased to 10.5with CaO and a small amount of sodium silicate was added prior to MgOaddition. After reaction, the sample was filtered and H₂O₂ test wasconducted as described above. The results are shown in Table 13. TABLE13 Peroxide Tests Results for the Pyrite Sample with Different Dosage ofMgO and Sodium Silicate. MgO Dosage Sodium Silicate Peroxide (lbs./T)Dosage (lbs./T) Test Result Remarks 13.2 0 Failed pH 3.70 after 3 hours17.6 0 Failed pH 4.30 after 3 hours 22.0 0 Passed Final pH after 1 day7.80 26.4 0 Passed Final pH after 1 day 8.30 17.6 4.4 Passed Final pHafter 1 day 7.81 22.0 4.4 Passed Final pH after 1 day 8.30

[0063] As can be seen in Table 13, with addition of only 17.6 lbs./tMgO, pyrite was not passivated. However, addition of 4.4 lbs./t sodiumsilicate in the presence of 17.6 lb/ton MgO increased the stability andthe pH remained about 7.81. It is evident that sodium silicate improvesthe passivation.

[0064] The effect of silicate addition is also demonstrated in FIG. 1.As can be seen, passivated pyrite samples with MgO and silicate in thepresence of lime showed improved resistance to peroxide oxidation ascompared to samples with no silicate.

[0065] Combined Effect of Calcium Silicate with MgO

[0066] In another series of experiments the combined effect of MgO withcalcium silicate was tested. The pH of the pyrite sample was increasedto 10.5 with CaO and a small amount of calcium silicate was added priorto MgO addition. After reaction, the sample was filtered and an H₂O₂test was conducted as above. The results are given in Table 14. TABLE 14Peroxide Test Results for the Pyrite Sample with Different Dosage of MgOand Calcium Silicate. (MgO Dosage, Calcium Silicate Peroxide Test lb/T)(lb/T) Result Remarks 13.2 0 Failed pH 3.7 after 3 hrs 17.6 0 Failed pH3.78 after 24 hrs 17.6 4.4 Passed pH 7.6 after 24 hours

[0067] As can be seen from Table 14, 14.4 lb/ton addition of calciumsilicate increased the passivation of pyrite. This shows that calciumsilicate can be used in conjunction with CaO to passivate pyrite at pH10.5.

REFERENCES

[0068] 1. Caruccio, F. T., Geidel, G., Pelletier, M., “Occurrence andpredication of acid drainage”. J. of the Energy Division, ASCE, 107, No.1, pp.167, 1981.

[0069] 2. De Vries, Nadine H. C. Process for Treating Iron-ContainingSulfide Rocks and Ores, U.S. Pat. No. 5,587,001, 1996.

[0070] 3. Doyle, F. M. and Mirza, A. H., “Understanding the mechanismsand kinetics of pyrite wastes”. Proceedings of the Western RegionalSymposium on Mining and Mineral Processing, Doyle, F. M. (eds.), Societyof Mining Engineering. 1990.

[0071] 4. Evangelou, V. P., “Pyrite Chemistry: The Key for Abatement ofAcid Mine Drainage”. Acidic Mining Lakes: Acid mine Drainage, Limnologyand Reclamation Springer-Verlag, 1998.

[0072] 5. Huang, X. and Evangelou, V. P., Abatement of acid minedrainage by encapsulation of acid producing geological materials, USBureau of Mines, Contract No. J0309013, 1992.

[0073] 6. Kleinmann, R. L. P., “Acid mine drainage: US Bureau of Minesresearches and develops control methods for both coal and metal mines”.Enviro. Mining J., July, pp161-164, 1989.

[0074] 7. Marshall, G. P., J. S. Thompson, and R. E. Jenkins, “Newtechnology for the prevention of acid rock drainage”. Proceedings of theRandol Gold and Silver Forum, pp. 203, 1998.

[0075] 8. Sobek, A. A., Schuller, W. A., Freeman, J. R., and Smith, R.M., Field and laboratory methods applicable to overburden mine soils.EPA 600/2-78-054, pp203, 1978.

[0076] In the disclosed process, as is generally true for otherprocesses, the fewer chemicals used, the more cost effective theprocess. If desired, other chemicals can be used in the disclosedprocess, including barium hydroxide and calcium carbonate for pHcontrol, but it is desired that as few chemicals as possible be used tolower the cost of the process.

[0077] All numerical ranges given herein include all useful intermediateranges and values thereof. Useful ranges and values may be determinedusing the teachings herein and those known in the art without undueexperimentation. Useful chemical equivalents may be used for thosechemicals specifically exemplified in this disclosure, as known by oneof ordinary skill in the art without undue experimentation.

[0078] All references cited herein are hereby incorporated by referenceto the extent not inconsistent with the disclosure herein. Although thedescription herein contains many specificities, these are not to beconstrued as limiting the scope of the invention, but as merelyproviding illustrations of some of the presently-preferred embodimentsof the invention. For example, the magnesium may be in the form ofmagnesium oxide, or other forms, as known in the art. Thus, the scope ofthe invention should be determined by the appended claims and theirlegal equivalents, rather than by the examples given.

We claim:
 1. A method for passivating sulfidic iron-containing rockcomprising: contacting sulfidic iron-containing rock with a magnesiumcontaining substance comprising one or more members of the groupconsisting of: magnesium oxide, magnesium hydroxide, magnesium chloride,magnesium nitrate and magnesium carbonate; adjusting the pH of thesystem to between about 9-11.
 2. The method of claim 1, furthercomprising adding a silicate.
 3. The method of claim 2, wherein thesilicate is selected from the group consisting of sodium and calciumsilicate.
 4. The method of claim 2, wherein the concentration ofsilicate is between 1-5 lb silicate/ton rock.
 5. The method of claim 1,further comprising adding air.
 6. The method of claim 1, furthercomprising adding an iron salt.
 7. The method of claim 6, whereinferrous iron-magnesium sulfates are formed.
 9. The method of claim 6,wherein said iron salt is FeCl₃.
 10. The method of claim 6, wherein theconcentration of iron salt added is sufficient to form the desiredamount of ferrous iron-magnesium sulfates.
 11. The method of claim 1,further comprising adjusting the pH of the system to below 5 before thecontacting step.
 12. The method of claim 1, wherein the magnesiumcontaining substance is magnesium oxide.
 13. The method of claim 11,wherein said magnesium oxide is present at a concentration of between2.2-22.0 lbs MgO/ton of rock.
 14. The method of claim 1, wherein themagnesium containing substance is magnesium hydroxide.
 15. The method ofclaim 13, wherein said magnesium hydroxide is present at a concentrationof about 2.5% by weight of solution.
 16. The method of claim 1, whereinsaid rock is present in a slurry of 20-50% by weight of solids.
 17. Themethod of claim 11, wherein the weight ratio of magnesium oxide: rock:water is maintained at 1:100:400-10:100:400.
 18. A method of reducingacid rock drainage from sulfidic iron-containing rock comprising thesteps of: contacting said rock with magnesium oxide wherein theconcentration of magnesium oxide in the mixture is 0.1-1% by weight andthe slurry density is about 20% by weight of solids in the mixture;allowing a reaction between said magnesium oxide and the sulfides insaid rock to proceed so as to form in slurry dissolved magnesiumsulfate; raising the pH of the slurry to about 10-10.5; adding silicateto said slurry.
 19. The method of claim 17, further comprisingcontacting said slurry with air.
 20. The method of claim 17, furthercomprising adding an iron-containing substance.
 21. A method of reducingacid rock drainage from sulfidic iron-containing rock comprising thesteps of: contacting said rock with an aqueous colloidal suspension of2.5% magnesium hydroxide; allowing a reaction between said magnesiumhydroxide and the sulfides in said rock to proceed so as to form inslurry dissolved magnesium sulfate; raising the pH of the slurry toabout 10-10.5; adding a silicate to the slurry.
 22. The method of claim21, further comprising contacting said slurry with air.
 23. The methodof claim 21, further comprising adding an iron-containing substance. 24.A method of passivating sulfidic iron-containing rock comprising:contacting sulfidic iron-containing rock with a member of the groupconsisting of: magnesium oxide, magnesium hydroxide, magnesium chloride,magnesium nitrate and magnesium carbonate at a concentration of betweenabout 25 moles Mg/ton of rock to 250 moles Mg/ton of rock and raisingthe pH to about 9-11.
 25. The method of claim 24, further comprisingadding one or more of sodium or calcium silicates at a concentration ofbetween 1-5 lb silicate/ton rock.
 26. The method of claim 24, furthercomprising oxidizing the rock.
 27. The method of claim 24, furthercomprising adding iron chloride.